Is that—on his school of law—viewed in two possible ways?
They explained the matter by noting that these two days are revered by the unbelievers; therefore, to single them out for fasting rather than other days would be to share with them in that reverence, and so it is disliked, just like fasting on Saturday. Imam Abū Muḥammad al-Maqdisī said: “By this same analogy, every festival of the unbelievers, or any day they set aside for exaltation, falls under the same ruling.”
It may also be said: fasting on the day of Nayruz, Mihrjan, and similar occasions whose dates are not determined by the Arab calendar is disliked, unlike what is reported concerning Saturday and Sunday. For if a person intentionally fasts on these Persian or pre-Islamic days, it becomes a means of establishing their emblem, reviving their observance, and displaying their significance. Saturday and Sunday, however, fall within the Muslims’ reckoning of days, and fasting them entails no such harm. Thus, reconciling the various reports would be to recommend fasting on those of their festivals that are reckoned by the Arab-Islamic calendar, while deeming it disliked to fast on those fixed by the pre-Islamic, foreign reckoning. Allah knows best.